Pulp and paper mills are under increased pressure to reduce both the quantity of water intake and the quantity of aqueous effluent discharged into the environment. The amount of water throughput has been significantly reduced in modern mills by employing a variety of techniques, such as countercurrent washing techniques for bleaching and utilization of stripped foul condensate from black liquor evaporators.
However, water discharged from pulp and paper mills typically includes elevated organic loads and a variety of other contaminants, such as color, fabric, and inorganic toxins. A number of methods have been employed to diminish the BOD (biological oxygen demand), COD (chemical oxygen demand), color, and solids content of effluent streams. Nevertheless, each of these methods has shortcomings, even when employed in combination with each other. For example, the color content of effluents has been significantly reduced by the addition of oxygen, or hydrogen peroxide in combination with oxygen. However, these treatments do not substantially change the BOD and COD of the effluent. In another example, clarifiers, or sedimentation tanks, have been employed to remove large amounts of solids from aqueous effluents. However, such facilities are subject to spillage and upsets which can cause the clarity of treated effluent to be significantly diminished.
Ultrafiltration and reverse osmosis have been shown to be effective in concentrating bleach plant effluents and in producing filtrates that are suitable for reuse in pulp and paper mills as clean wash water. However, these methods of filtration typically produce a concentrate of organic solids which have a high BOD and include toxic inorganic and organic components.
Therefore, a need exists for a method and system which overcomes or minimizes the above-mentioned problems.